Hi Roper - I think you may be referring the L designation given to standard blanks, such as Cartridge S.A. Blank .303 inch Cordite without Bullet Mark V. On purpose made cases, the head stamp would read MF 44 LV (for example).
Grenade launching and line throwing blanks were given the letter designation H, such Cartridge SA Rifle Grenade .303 inch H Mark II. On purpose made cases, the head stamp would read MF 42 HII (for example).
Because any reject case can be used for blanks, a variety of headstamps from any other loading can be used, which can become a little confusing. I have ball, trace, AP and incendiary cases that have been loaded as blanks. Please correct me if I misinterpreted your meaning.